1. Field of the Invention
This invention relates to a pseudo-random number sequence output unit, transmitter, receiver, communication system and filter unit, a pseudo-random number sequence output method, transmission method and receiving method, and a data recording medium.
This invention particularly relates to an output unit and output method suitable for outputting pseudo-random number sequences usable as the spreading codes of an asynchronous CDMA (Code Division Multiple Access) system for satellite, point-to-point, mobile phone and PHS (Personal Handyphone System) communication systems and other land mobile communication systems, and in GPS (Global Positioning System) and other distance measurement fields; a transmitter, receiver, communication system, filter unit, transmission method, receiving method and filtering method using the spreading codes; and a computer-readable data recording medium recorded with a program for implementing any of the foregoing.
2. Description of the Prior Art
Spreading codes developed for enabling spread-spectrum communication systems and code division multiple telecommunication include M sequences, Kasami sequences and Gold sequences generated by an LFSR (Linear Feedback Shift Register). These spreading code sequences have the following two characteristics.
First, the auto-correlation function of codes has a peak and the correlation between different codes (cross-correlation) is near 0. This is very similar to the property of white noise.
Second, when two different spreading codes contained in a code set are selected and the code set is constituted such that the cross-correlation is near 0 regardless of which are selected, the number of codes contained in the code set is small relative to the code length. The number of code types is therefore few.
On the other hand, TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) have been known for many years. The asynchronous CDMA communication system differs from these in its feature of enabling despreading by use of the correlation characteristic of the used codes even without positive signal synchronization. It is therefore superior in privacy, secrecy, anti-interference property, anti-jamming property and the like.
Efforts are being made to put CDMA communication into practical use. IMT-2000 (International Mobile Telecommunication 2000), a next-generation wireless telecommunication ITU (International Telecommunication Union) standard, has been selected for adoption.
Recent research shows that the performance of an asynchronous CDMA communication system is determined by inter-code interference noise variance σ. When pseudo-white noise type spreading codes like Gold sequences or Kasami sequences are used, the interference noise variance σ is asymptotically equal to (K−1)/3N, where K is the number of simultaneously connected users and N is the code length. (M. B. Pursley, “Performance Evaluation for Phased-Coded Spread-Spectrum Multiple-Access Communication—Part I: System Analysis,” IEEE Trans. Communication, vol. 25 (1977) pp. 795-799.)
“Asymptotical” here refers to the case where the user number (number of users) K and the code length N have become large.
The theoretical limit of asynchronous CDMA communication system performance has been considered to be σ=(K−1)/3N. It is known, however, that the asymptotic relationship holds because the spreading codes are assumed to be pseudo-white noise.
Therefore, when the spreading codes are not pseudo-white noise, i.e., when some degree of correlation is present between different codes, the theoretical limit of performance can be improved.
Spreading codes have recently been discovered that have an auto-correlation function whose inter-code interference noise variance σ is lower than that when the spreading codes are pseudo-white noise. Specifically, when the auto-correlation function decreases exponentially with a code shift of 1 in the manner of Eq. (1), the interference noise dispersion σ is smaller than in the case of pseudo-white noise.C(l)≈Const.×(−r)l (−1<r<1)  (1)
In particular, the optimal correlation function (3) is obtained when the real impulse constant r satisfies Eq. (2).γ≈2−√{square root over (3)} (must be an equality)  (2)
                              σ          optimal                =                                            3                        ⁢                          (                              K                -                1                            )                                            6            ⁢            N                                              (        3        )            
This means that at the same bit error rate the number of simultaneously connected users K is 15% greater than the theoretical limit number of users of an asynchronous CDMA communication system using pseudo-white noise as the spreading codes. (G Mazzini, R. Rovatti, G Setti, “Interference Minimization by Auto-correlation Shaping in Asynchronous DS-CDMA Systems: Chaos-based Spreading is Nearly Optimal,” IEE Electronic Letters, vol. 35, n. 13, Jun. 24 1999, pp. 1054-1055).
This paper also points out that a correlation function satisfying Eq. (1) can be approximately modeled by generating chaos spreading codes by piecewise-linear maps of very large partial slope.
When an attempt is made to generate such spreading sequences with a DSP (Digital Signal Processor) or the like and utilize them in a mobile phone system, for example, the following problems arise owing to the need for high-speed and low power consumption.
First, owing to the fact that the spreading codes are generated by piecewise-linear maps of very large slope, accurate results cannot be obtained by DSP implementation or computer calculation because of high digit dropout. This makes it difficult to construct a physical circuit or device for generating the spreading codes.
Second, the piecewise-linear maps with the parameter that determines how the correlation function attenuates cannot be freely designed with respect to an arbitrary r (−1<r<1).
Third, the Mazzini et al. paper points out that few types of the piecewise-linear maps have correlation functions near optimal. It is, however, desirable to have as many types of codes as possible for realizing a CDMA communication system. Actual configuration of a CDMA communication system using the method taught by this paper is therefore difficult.
Fourth, in the case of spreading codes generated using a linear shift register, only 0(N) types of codes having a good correlation characteristic with respect code length N are available. This is very few relative the original number of code types, which is proportional to the power of 2 O(2N). It is therefore difficult to cope with an increase in the number of users.
Fifth, the small key space makes decoding possible with little time or trouble. Communication security is therefore poor.
The teachings of this paper offer no remedy for any of these five problems.
A need has therefore been strongly felt for a technology for overcoming these problems that is capable of generating spreading codes consisting of pseudo-random number sequences (also called PN (Pseudo-Noise) sequences) suitable for an asynchronous CDMA communication system.
An object of the present invention is to provide a pseudo-random number sequence output unit, transmitter, receiver, communication system and filter unit, and a pseudo-random number sequence output method, transmission method, receiving method and filtering method that are suitable for an asynchronous CDMA communication system, and a data recording medium recorded with a program for implementing any of the foregoing.